This invention relates to a keyboard switching assembly in the form of a variable capacitor or condenser capable of producing an electrical output through variation in capacitance thereof, and a process of manufacturing the switching assembly, more particularly, a movable electrode thereof.
Various forms of keyboard switching devices of different constructions and characteristics have been proposed for producing electrical outputs which correspond to data input keys on a keyboard which have been depressed or operated. For example, there have been marketed and used magnetically operated reed switches (combination of ferromagnetic reeds and a permanent magnet), switches having gold-plated contacts, and other contact-equipped switches wherein contacts are mechanically operated to make and break a circuit. Contactless keyboard switches have also been developed making use of magnet- or pressure-sensitive elements. While the contactless switches in the form of a variable capacitor are considerably advantageous in cost of materials over the contact-equipped switches and the contactless switches using the magnet- or pressure-sensitive elements, they have not been found completely satisfactory in performance in terms of operating accuracy, durability, and reliability.
There has been thus developed a need for a keyboard switching device or assembly of variable capacitance type which is improved in performance or capability. For example, a capacitive switch is known according to Japanese Patent Application No. 48-21592 (filed on Feb. 22, 1973, laid open as Publication No. 49-109872 on Oct. 18, 1974), wherein a movable electrode is formed of a spongy material, polyester films bonded under heat to opposite surfaces of the spongy material with heat-sensitive adhesive layers coated on said opposite surfaces, an evaporated aluminum film adhering to one of the polyester films with a coating of mucilage or glue, and a sheet of release paper attached to the other polyester layer with another coating of the same mucilage or glue, which release paper does not permit the mucilage or glue to permeate thereinto, and is removed when the movable electrode is secured to a movable support member of a switching device. The polyester films bonded with the adhesive layers having no permeability after they have been cured, will function to prevent the used mucilage or glue from penetrating therethrough into the interior of the spongy material. Thus, the spongy material is not hardened by otherwise possible penetration of the mucilage or glue, and therefore the movable electrode is operable in good physical conditions for a longer period of service, contributing to improvement in performance of the capacitive switch.
However, it has been found time-consuming and cumbersome to bond such polyester films under heat to upper and lower surfaces of the spongy material, and technically difficult to obtain an overall thickness of the movable electrode with sufficiently high accuracy within a predetermined tolerance. Thus, the known keyboard switching devices in the form of a variable capacitor still suffer drawbacks as described hereinbefore.